With the progress in the technology of multi-layer printed circuit boards for a higher level of integration and size reduction, the use of bonding film has come to be preferred over more conventional glass cloth and other base materials because of ease of handling. The bonding film typically consists of organic material such as polyimide resin and epoxy resin. In particular, materials essentially consisting of epoxy resin are preferred in view of heat resistance, molding temperature and cost. Japanese patent laid open publication (kokai) No. 4-119846 discloses epoxy resin film consisting of high polymer epoxy resin. Japanese patent laid open publication (kokai) No. 4-339852 discloses an epoxy resin composition which consists of high polymer epoxy resin, polyfunctional epoxy resin, and a curing agent, and is suitable for making epoxy bonding film. Japanese patent laid open publication (kokai) No. 5-93041 discloses flame retardant epoxy resin film.
By combining polyfunctional epoxy resin with high polymer epoxy resin, it is possible to obtain bonding film for printed circuit boards that demonstrates high heat resistance, favorable bonding property, and high resistance to chemical erosions. However, bonding film containing polyfunctional epoxy and a curing agent gives rise to crosslinking by heating, and is known to lose some of the flexibility once it is cured. Therefore, when such bonding film is used as a cover layer of a printed circuit board having a flexible section, cracks may be produced in parts of the bonding film when such a flexible section is bent due to small scratches and other damages that may be sustained during the process of washing, grinding, cutting, transportation, and mounting component parts. It is also known that the flexibility may be improved by adding rubber-like resin in the bonding film, however, at the expense of compromising the flame retardant property of the bonding film.
Also, due to the fact that polyfunctional epoxy resin having a relatively small molecular weight, the bonding film in its semi-cured state may be so brittle that debris in the form of powder tends to be produced when cutting the bonding film. The tendency of the bonding film to easily crack makes the handling of the bonding film difficult.
The Inventors of this patent application has studied these problem, and has discovered that the bonding film prepared according to the present invention can resolve such problems.